The invention relates generally to apparatus and methods useful in detecting the presence of debris and contamination in fluids. More particularly, the invention relates to the detection and removal of magnetic debris from fluids, especially in applications where both the quantity and rate of debris accumulation in the fluid are important information. While the invention described herein is explained in connection with a lubricating oil type fluid, it will be appreciated that the invention is similarly useful in any liquid or gaseous fluid medium, or combination thereof.
The presence of contamination and debris in lubricating fluids such as machine and engine oils is clearly undesirable in most applications due to the attendant decrease in the fluid's capacity to protect moving parts from friction and wear. More significant, however, is that the presence of magnetic debris such as pieces of ferrous metal can be an indication of wear and damage in moving parts within the equipment. Early detection of such debris can be critical in applications such as gas turbine engines used in today's advanced aircraft. As the state of the art advances for such engines, failure rates tend to increase for the high performance engine parts. The ability to detect the onset of failure of moving engine components is imperative not only in military aircraft with high thrust to weight ratios, but also commercial aircraft with turbine engines that are operated for long periods of time. Early detection of critical part wear reduces repair costs, secondary engine damage, and unscheduled maintenance time. Of course, such early detection can be useful in many other applications besides aircraft engines where failure of moving lubricated parts results in significant repair costs and down time.
Prior attempts at detecting and capturing debris in fluids include electric chip detectors and magnetic chip detectors. Typically, such detectors are placed in the fluid returning from the engine in an oil scavenge line. Magnetic chip detectors are essentially just magnets that capture magnetic debris in the fluid. There is no real-time feedback available on the aircraft to indicate the status of the detector. Rather, during maintenance the magnetic chip detector must be removed for visual inspection.
Electric chip detectors function similarly to the magnetic chip detectors, except they further include an electrical continuity indication capability. Typically, two electrodes are positioned near a magnet such that captured particles eventually bridge a gap between the electrodes causing a change in continuity. While this approach can provide a rudimentary feedback signal to the cockpit, little or no quantitative or qualitative information can be discerned from such a signal. This approach also does not include any trending capability. The data does not provide an indication of the quantity of material accumulated, nor the rate of accumulation. Knowing the rate of accumulation is a highly desirable feature because the rate of debris accumulation in the fluid is an excellent indication of the severity and rate of failure of the machine or engine par&:s. The electric chip detectors are also very susceptible to false indications due to spurious noise, background debris and electromagnetic interference.
The magnetic chip detectors, as well as the electric chip detectors, rely primarily on laboratory analysis of the collected debris to determine the condition of the equipment being monitored. That is, measurement of the debris particles and analysis of their total accumulation usually only occurs when maintenance personnel remove and visually inspect the sensors. The frequent inspection requirement, of course, is not only time consuming and costly, but also increases the likelihood of mistakes being made during teardown and reassembly. The lack of a real-time analysis is a significant drawback because substantial deterioration can occur between scheduled maintenance activities. Other types of oil debris monitoring systems are discussed in SAE publication AIR 1828 dated Mar. 1, 1984. Such systems include inductive debris monitors that produce electric pulses when debris contacts coils, and electro-optical monitors that use the scattering of light by debris in the fluid. These systems are still developmental and are subject to false indications and noise.
A known system that purports to provide both particle quantity and rate of occurrence on a real-time basis is described in U.S. Pat. No. 4,219,805 issued to Magee et al. This system detects large particles of two predetermined sizes (particles greater than 250 microns and particles greater than 1000 microns) by generating electric pulses upon impact of the particles with the magnetic sensor. These electric pulses are counted by an electronic analyzer to approximate both the quantity of accumulation and the rate of accumulation. The data can be recorded on a chart recorder for trend analysis during maintenance activity, and can also be viewed on a realtime basis to detect rapid catastrophic failures. The system, however, is still susceptible to spurious noise and electromagnetic interference near the sensor. Moreover, the need to constantly monitor electric pulses at the sensor output requires expensive and complicated electronic hardware. When multiple sensors are required, a situation that is quite common in aircraft applications, each sensor must be individually and continually monitored to avoid missing the effects of a particle impacting the sensor. This makes data multiplexing costly and difficult to implement. The system also ignores failure modes indicated by increased production of debris particles that are too small to trigger the sensor.
The use of optical transducers to detect variations in the intensity of a magnetic field modulated by a moving target is well known. One such application is detecting a rotating target to discern rotational speed. One such transducer is shown in U.S. Pat. No. 4,947,035 issued to Zook et al. The conventional features of earlier optical transducers are the use of sensor light propagating through a magneto-optic material in combination with a magnet or other source of a magnetic field such as can be induced by electric current in a conductor. In a magneto-optic material, the polarization state of light propagating through the material is rotated through an angle proportional to the magnetic field. This angular rotation in the light's polarization state results from the effects of an electromagnetic phenomena known as the Faraday Effect. Rotation of the sensor light's angle of polarization is then converted into an amplitude modulation of the light intensity through the use of one or more polarizers. This amplitude modulation of light can then be detected by the use of electro-optic transducers such as photodiodes and similar devices that produce electrical signals proportional to the intensity of incident light thereon.
Because the basic operation of optical transducers utilizing the Faraday Effect depends on amplitude modulated intensity signals, the accuracy of the information contained in the modulated signal is highly dependant on the intensity of the light source, the sensitivity of the light detectors, and changes in light intensity due to variations in the optical elements through which the light travels. Accordingly, prior systems relied on the use of a reference light beam that has a wavelength that is different from the wavelength of the sensor light and typically has a wavelength outside the active bandwidth of the magneto-optic material. By having a reference light beam with a different wavelength from that of the modulated beam, the reference beam can be transmitted along a similar optical path as the modulated beam but is unaffected by the magneto-optic material. The use of this referencing scheme, however, is complicated and expensive because different wavelength light sources are required, special processing of the magneto-optic material can be required, and the sensor itself may need to have secondary optical paths to transmit the reference beam. Also, intensities of the different sources can still change with time and temperature and affect accuracy.
For these and other reasons that will be apparent hereinbelow there exists a need for a debris monitoring system that is both accurate and cost-effective to use and that provides a real-time analysis of the accumulation of magnetic debris in a fluid. There also exists the need for an inherently self-referenced optical sensor that can detect a variable magnetic field with high accuracy.